About juice storage, general consensus is that your juices should be good for at least a year if stored in a cool place. The juice won't turn to poison or anything, but the nicotine does degrade eventually and the juice's nic level will start to fall. This is a pretty slow process, though.
Juice can change color because of ongoing diffusion of the ingredients, slow chemical reaction between the juice's ingredients, or oxidation (exposure to air turns nicotine increasingly brown over time). Sometimes this improves the juice, but mostly it doesn't affect the taste. The general quality and potency is unaffected.
Also, juice won't mold up or anything. pg itself is a pretty good preservative, and there's not much there for mold to grow on anyway.
Tips for stealth vaping, take a draw a bit shorter than normal and continue to inhale clean air afterward. Hold your breath for about 3 seconds then exhale, there should be nearly no vapor visible, if the vapor is still visible purse your lips like you are going to whistle crating a very small hole between your lips and exhale a bit harder than normal so you can feel a bit of pressure in your mouth, that will make any remaining vapor invisible.
Or, you can just hold your breath longer! I find about 15-20 seconds, and I'm exhaling no vapor whatsoever. I draw normally, not short, but I make sure to inhale some clean air through my nose and mouth, as vapor left in there doesn't dissipate quickly enough and if I don't "flush", I'll emit a single small puff of tell-tale vapor when I exhale.